Quantification of lectin fluorescence in GNE myopathy muscle biopsies

GNE myopathy: History, etiology, and treatment trials

GNE myopathy is an ultrarare muscle disease characterized by slowly progressive muscle weakness. Symptoms typically start in early adulthood, with weakness and atrophy in the tibialis anterior muscles and with slow progression over time, which largely spares the quadriceps muscles. Muscle biopsy shows atrophic fibers and rimmed vacuoles without inflammation. Inherited in an autosomal recessive manner, patients with GNE myopathy carry mutations in the GNE gene which affect the sialic acid synthesis pathway. Here, we look at the history and clinical aspects of GNE myopathy, as well as focus on prior treatment trials and challenges and unmet needs related to this disorder.

Recent advances in establishing a cure for GNE myopathy

PURPOSE OF REVIEW

GNE myopathy is a rare autosomal recessive disease caused by biallelic variants in the GNE gene, which encodes an enzyme involved in sialic acid biosynthesis. No drugs are approved for the treatment of GNE myopathy. Following proof-of-concept of sialic acid supplementation efficacy in mouse models, multiple clinical trials have been conducted. Here, we review clinical trials of sialic acid supplementation therapies and provide new insights into the additional clinical features of GNE myopathy.

RECENT FINDINGS

Clinical trials of sialic acid supplementation have been conducted in Europe, the USA, Japan, and South Korea. Some clinical trials of NeuAc-extended release tablets demonstrated amelioration of decline in upper extremity muscle strength; however, no significant improvement was observed in phase 3 trials in Europe and USA. A phase 2 trial of ManNAc showed slowed decline of both upper and lower extremity strength. GNE myopathy patient registries have been established in Europe and Japan, and have provided information on extramuscular manifestations such as thrombocytopenia, respiratory dysfunction, and sleep apnea syndrome. Sensitive and reliable biomarkers, and a disease-specific functional activity scale, have also been investigated.

SUMMARY

We discuss recent advances in establishing a GNE myopathy cure, and discuss other prospective therapeutic options, including gene therapy.

The role of amyloid β in the pathological mechanism of GNE myopathy

GNE myopathy is a hereditary muscle disorder characterized by muscle atrophy and weakness initially involving the lower distal extremities. The treatment of GNE myopathy mainly focuses on a sialic acid deficiency caused by a mutation in the GNE gene, but it has not achieved the expected effect. The main pathological features of GNE myopathy are myofiber atrophy and rimmed vacuoles, including accumulation of amyloid β, which is mainly found in atrophic muscle fibers. Although the role of amyloid β and other misfolded proteins on the nervous system has been widely recognized, the cause and process of the formation of amyloid β in the pathological process of GNE myopathy are unclear. In addition, amyloid β has been reported to be linked to quality control mechanisms of proteins, such as molecular chaperones, the ubiquitin–proteasome system, and the autophagy-lysosome system. Herein, we summarize the possible reasons for amyloid β deposition and illustrate amyloid β-mediated events in the cells and their role in muscle atrophy in GNE myopathy. This review represents an overview of amyloid β and GNE myopathy that could help identify a potential mechanism and thereby a plausible therapeutic for the disease.

Safety and efficacy of N-acetylmannosamine (ManNAc) in patients with GNE myopathy: an open-label phase 2 study

Purpose

To evaluate the safety and efficacy of N-acetylmannosamine (ManNAc) in GNE myopathy, a genetic muscle disease caused by deficiency of the rate-limiting enzyme in N-acetylneuraminic acid (Neu5Ac) biosynthesis.

Methods

We conducted an open-label, phase 2, single-center (NIH, USA) study to evaluate oral ManNAc in 12 patients with GNE myopathy (ClinicalTrials.gov NCT02346461). Primary endpoints were safety and biochemical efficacy as determined by change in plasma Neu5Ac and sarcolemmal sialylation. Clinical efficacy was evaluated using secondary outcome measures as part of study extensions, and a disease progression model (GNE-DPM) was tested as an efficacy analysis method.

Results

Most drug-related adverse events were gastrointestinal, and there were no serious adverse events. Increased plasma Neu5Ac (+2,159 nmol/L, p < 0.0001) and sarcolemmal sialylation (p = 0.0090) were observed at day 90 compared to baseline. A slower rate of decline was observed for upper extremity strength (p = 0.0139), lower extremity strength (p = 0.0006), and the Adult Myopathy Assessment Tool (p = 0.0453), compared to natural history. Decreased disease progression was estimated at 12 (γ = 0.61 [95% CI: 0.09, 1.27]) and 18 months (γ = 0.55 [95% CI: 0.12, 1.02]) using the GNE-DPM.

Conclusion

ManNAc showed long-term safety, biochemical efficacy consistent with the intended mechanism of action, and preliminary evidence clinical efficacy in patients with GNE myopathy.

Population Pharmacokinetic Model of N-acetylmannosamine (ManNAc) and N-acetylneuraminic acid (Neu5Ac) in Subjects with GNE Myopathy

BACKGROUND

GNE myopathy is a rare genetic muscle disease resulting from deficiency in an enzyme critical for the biosynthesis of N-acetylneuraminic acid (Neu5Ac, sialic acid). The uncharged Neu5Ac precursor, N-acetylmannosamine (ManNAc), is under development as an orphan drug for treating GNE myopathy.

METHODS

A semi-mechanistic population pharmacokinetic model was developed to simultaneously characterize plasma ManNAc and its metabolite Neu5Ac following oral administration of ManNAc to subjects with GNE myopathy. Plasma ManNAc and Neu5Ac pharmacokinetic data were obtained from two clinical studies (ClinicalTrials.gov identifiers NCT01634750, NCT02346461) and were simultaneously modeled using NONMEM.

RESULTS

ManNAc and Neu5Ac plasma concentrations were obtained from 34 subjects with GNE myopathy (16 male, 18 female, median age 39.5 years). The model parameter estimates included oral absorption rate (k_a) = 0.256 h^-1, relative bioavailability relationship with dose (F-Dose) slope = -0.405 (where F = 1 for 6-g dose), apparent clearance (CL_M/F) = 631 L/h, volume of distribution (V_M/F) = 506 L, Neu5Ac elimination rate constant (k_out) = 0.283 h^-1, initial ManNAc to Neu5Ac conversion (SLP₀) = 0.000619 (ng/mL)^-1 and at steady-state (SLP_SS) = 0.00334 (ng/mL)^-1, with a rate-constant of increase (k_inc) = 0.0287 h^-1. Goodness-of-fit plots demonstrated an acceptable and unbiased fit to the plasma ManNAc and Neu5Ac concentration data. Visual predictive checks demonstrated reasonable agreement between the 5th, 50th, and 95th percentiles of the observed and simulated data.

CONCLUSIONS

This population pharmacokinetic model can be used to evaluate ManNAc dosing regimens and to calculate Neu5Ac production and exposure following oral administration of ManNAc in subjects with GNE myopathy.

Congenital thrombocytopenia associated with GNE mutations in twin sisters: a case report and literature review

Background

Neonatal thrombocytopenia is common in preterm and term neonates admitted to neonatal intensive care units. The etiology behind neonatal thrombocytopenia is complex. Inherited thrombocytopenia is rare and usually results from genetic mutations.

Case presentation

Here we report a case of twins with severe inherited thrombocytopenia presented in the neonatal period who were shown to be compound heterozygotes for 2 UDP-N-acetylglucosamine 2-epimerase (GNE) gene mutations, c.1351C > T and c.1330G > T, of which c.1330G > T is a novel mutation.

Conclusion

These two GNE mutations may help in the diagnosis and management of thrombocytopenia diagnosed in neonates.

GNE Myopathy: Etiology, Diagnosis, and Therapeutic Challenges

GNE myopathy, previously known as hereditary inclusion body myopathy (HIBM), or Nonaka myopathy, is a rare autosomal recessive muscle disease characterized by progressive skeletal muscle atrophy. It has an estimated prevalence of 1 to 9:1,000,000. GNE myopathy is caused by mutations in the GNE gene which encodes the rate-limiting enzyme of sialic acid biosynthesis. The pathophysiology of the disease is not entirely understood, but hyposialylation of muscle glycans is thought to play an essential role. The typical presentation is bilateral foot drop caused by weakness of the anterior tibialis muscles with onset in early adulthood. The disease slowly progresses over the next decades to involve skeletal muscles throughout the body, with relative sparing of the quadriceps until late stages of the disease. The diagnosis of GNE myopathy should be considered in young adults presenting with bilateral foot drop. Histopathologic findings on muscle biopsies include fiber size variation, atrophic fibers, lack of inflammation, and the characteristic "rimmed" vacuoles on modified Gomori trichome staining. The diagnosis is confirmed by the presence of pathogenic (mostly missense) mutations in both alleles of the GNE gene. Although there is no approved therapy for this disease, preclinical and clinical studies of several potential therapies are underway, including substrate replacement and gene therapy-based strategies. However, developing therapies for GNE myopathy is complicated by several factors, including the rare incidence of disease, limited preclinical models, lack of reliable biomarkers, and slow disease progression.